Even though the automotive industry has over the years, if for no other reason than seeking competitive advantages, continually exerted efforts to increase the fuel economy of automotive engines, the gains continually realized thereby have been deemed by various levels of government as being insufficient. Further, such levels of government have also arbitrarily imposed regulations specifying the maximum permissible amounts of carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NO.sub.x) which may be emitted by the engine exhaust gases into the atmosphere.
Unfortunately, generally, the available technology employable in attempting to attain increases in engine fuel economy is contrary to that technology employable in attempting to meet the governmentally imposed standards on exhaust emissions.
The prior art is trying to meet the standards for NO.sub.x emissions has employed a system of exhaust gas recirculation whereby at least a portion of the exhaust gas is reintroduced into the cylinder combustion chamber to thereby lower the combustion temperature therein and consequently reduce the formation of NO.sub.x.
The prior art has also proposed the use of engine crankcase recirculation means whereby the vapors which might otherwise become vented to the atmosphere are introduced into the engine combustion chambers for further burning.
The prior art has also proposed the use of fuel metering means which are effective for metering a relatively overly rich (in terms of fuel) fuel-air mixture to the engine combustion chamber means as to thereby reduce the creation of NO.sub.x within the combustion chamber. The use of such overly rich fuel-air mixtures results in a substantial increase in CO and HC in the engine exhaust which, in turn, requires the supplying of additional oxygen, as by an associated air pump, to such engine exhaust in order to complete the oxidation of the CO and HC prior to its delivery into the atmosphere.
The prior art has also heretofore proposed employing the retarding of the engine ignition timing as a further means for reducing the creation of NO.sub.x. Also, lower engine compression ratios have been employed in order to lower the resulting combustion temperature within the engine combustion chamber and thereby reduce the creation of NO.sub.x. In this connection the prior art has employed what is generally known as a dual bed catalyst. That is, a chemically reducing first catalyst is situated in the stream of exhaust gases at a location generally nearer the engine while a chemically oxidizing second catalyst is situated in the stream of exhaust gases at a location generally further away from the engine and downstream of the first catalyst. The relatively high concentrations of CO resulting from the overly rich fuel-air mixture are used as the reducing agent for NO.sub.x in the first catalyst while extra air supplied (as by an associated pump) to the stream of exhaust gases, at a location generally between the two catalysts, serves as the oxidizing agent in the second catalyst. Such systems have been found to have various objections in that, for example, they are comparatively very costly requiring additional conduitry, air pump means and an extra catalyst bed. Further, in such systems, there is a tendency to form ammonia which, in turn, may or may not be reconverted to NO.sub.x in the oxidizing catalyst bed.
The prior art has also proposed the use of fuel metering injection means for eliminating the usually employed carbureting apparatus and, under superatmospheric pressure, injecting fuel through individual injector nozzles directly into the respective cylinders of a piston type internal combustion engine. Such fuel injection systems, besides being costly, have not proven to be generally successful in that the system is required to provide metered fuel flow over a very wide range of metered fuel flows. Generally, those prior art injection systems (especially those employing injection nozzles with moving pintles or the like) which are very accurate at one end of the required range of metered fuel flows, are relatively inaccurate at the opposite end of that same range of metered fuel flows. Also, when such prior art injection systems are made to be accurate in the mid-portion of the required range of metered fuel flows are usually relatively inaccurate at both ends of that same range. The use of feed-back means for altering the metering characteristics of such prior art fuel injection systems has not solved the problem of inaccurate metering because the problem usually is intertwined within such factors as: effective aperture area of the injector nozzle; comparative movement required by the associated nozzle pintle or valving member; inertia of the nozzle valving member; and nozzle "cracking" pressure (that being the pressure at which the nozzle opens). As should be apparent, the smaller the rate of metered fuel flow desired, the greater becomes the influence of such factors thereon.
The prior art, in view of such anticipated requirements with respect to NO.sub.x, has suggested the employment of a "three-way" catalyst, in a single bed, within the stream of exhaust gases as a means of attaining such anticipated exhaust emission limits. Generally, a "three-way" catalyst is a single catalyst, or catalyst mixture, which catalyzes the oxidation of hydrocarbons and carbon monoxide and also the reduction of oxides of nitrogen. It has been discovered that a difficulty with such a "three-way" catalyst system is that if the fuel metering is too rich (in terms of fuel), the NO.sub.x will be reduced effectively but the oxidation of CO will be incomplete; if the fuel metering is too lean, the CO will be effectively oxidized but the reduction of NO.sub.x will be incomplete. Obviously, in order to make such a "three-way" catalyst system operative, it is necessary to have very accurate control over the fuel metering function of associated fuel metering supply means feeding the engine. As hereinbefore described, the prior art has suggested the use of fuel injection means, employing respective nozzles for each engine combustion chamber, with associated feedback means (responsive to selected indicia of engine operating conditions and parameters) intended to continuously alter or modify the metering characteristics of the fuel injection means. However, as also hereinbefore indicated, such fuel injection systems have not proven to be successful.
It has also heretofore been proposed to provide a fuel metering valving assembly which is electrically operated in response to electronically sensed signals. In such an electronic fuel metering valve assembly, the valve member is generally reciprocatingly moved toward and away from a cooperating metering orifice thereby correspondingly closing and opening such orifice to the flow of fuel therethrough. The percentage of time, within any selected span of time, in which the valve member is away from the orifice determines, in effect, the rate of flow therethrough. That is, the greater the percentage of time, the greater is the rate of metered fuel flow.
Such electronic fuel metering valve assemblies have been employed to meter fuel to the associated engine over a great range of engine speed and load conditions. Some engines, because of their design and or engine accessories, require a dramatically high rate of metered fuel flow as, for example, during maximum acceleration (maximum load) conditions. In such situations, of course, the metering orifice of the electronic fuel metering valve assembly has to be of a size sufficient to permit the maximum fuel flow required by the engine. However, as a consequence of this, it has been discovered that although the metering characteristics of the electronic fuel metering valve assembly are accurate during most of the entire range of required rates of metered fuel flow, it becomes somewhat inaccurate at the very low range of rate of metered fuel flow as where the relatively very large area metering orifice is being opened for a very short time by the cooperating valve member.
Accordingly, the invention as disclosed, described and claimed is directed, primarily, to the solution of such and other related and attendant problems of the prior art.